1. Field of the Invention
The present invention relates to a system for controlling the oil pressure of a frictional engagement element in a vehicular automatic transmission and, more particularly, to a system for controlling the oil pressure of a frictional engagement element participating in a downshift.
2. Description of the Prior Art
A particular shift of an automatic transmission may be executed by disengaging one frictional engagement element and engaging another frictional engagement element. This shifting mode is known as the so-called "clutch-to-clutch shift" . For this shift, it is necessary to take a proper synchronization between the engagement and disengagement of each of the frictional engagement elements.
If the engagement and disengagement of those frictional engagement elements should go out of timing, it may occur that the output shaft torque temporarily drops or that the engine is encountered by the blow-up, in which its revolution number abruptly increases for a while.
For this control, therefore, it is generally the current practice of the prior art that a one-way clutch for performing a function substantially similar to that of one of the frictional engagement elements is provided to effect the synchronization between the engagement and disengagement of the frictional engagement elements.
Since, however, the one-way clutch is limited in the direction to transmit the torque, nondirectional frictional engagement elements such as multi-disc clutches or brakes have to be additionally arranged in parallel with that one-way clutch. This necessity for the one-way clutch causes problems such as a rise of the production cost or an increase in the weight or the required space.
In recent years, various sensor technologies or electronic control technologies for the oil pressure control system have made remarkable progress. Trials for executing the clutch-to-clutch shift directly by using not any one-way clutch but those technologies are activated again.
In case a downshift is to be executed by resorting to the clutch-to-clutch shift, it is important to time the engagement a frictional engagement element at a lower gear stage after the downshift. Specifically, the frictional engagement element at the lower gear stage has to be engaged without fail at the instant when the rotations of predetermined rotary members are synchronized by disengaging the frictional engagement element which was engaged at a higher gear stage before the downshift. This technology for executing the downshift satisfactorily is disclosed in Japanese Patent Laid-Open No. 295060/1989. According to the technology disclosed in the Laid-Open, the synchronization between predetermined rotary members at a shifting time is decided on the basis of the input and output revolution numbers of an automatic transmission, and the oil pressure of a frictional engagement element to engage at a lower gear stage is abruptly raised after the synchronization has been decided.
Since, however, the oil pressure control system has inevitable response delays or dispersions, it is the current practice that the aforementioned control technology finds it actually difficult to engage the frictional engagement element at the lower gear stage without fail at the instant of the synchronization and that the engagement is liable to go out of timing. Moreover, the output shaft torque abruptly drops if the engagement of the frictional engagement element at the lower gear stage is timed premature, but the engine blows up if the engagement is timed late.